It is common engineering practice to design large assemblies with intentional gaps left between subassemblies to avoid interference and locked-in stresses in the final structure. Unintentional gaps are also formed due to lack of precise measurements. These gaps, referred to as shim gaps, whose surfaces may be nonparallel and irregular, are subsequently filled with shims that must be custom fitted to the gap in order to provide proper alignment and smooth load transfer. The quality of the shim used to fill the gap varies according to the requirements of the final structure. For example, in load-bearing joints, the shim material must match the physical characteristics of the parts to be joined. This means metal shims are required for metal structures, with the specific shim material depending on the application.
An additional problem encountered in attempting to measure a shim gap is that the clearance may be extremely limited and the range of variability very large, relatively speaking. In extreme cases, one could be called upon to measure a gap of 0.25 millimeter in one joint, and at the next joint measure a gap of 5 millimeters using the same gauge. Thus, ideally, a shim probe must be thin enough for insertion into an extremely thin gap, but capable of measuring a gap that is relatively large. This problem could be overcome by using probes with a variety of measurement range limitations. However, the use of more than one probe leads to inefficient production.
The range of tolerance can substantially affect the alignment and internal stress generation. This is an additional consideration in shim manufacturing. Suitable tolerances are determined by gap size and application. For example, for a shim gap ranging from 0.25 to 5 mm, an acceptable tolerance for airplane manufacture is generally .+-.0.1 mm.
Due to the number and variety of shim fittings that must be made in large structures, such as airplanes, an automated method of gauging the shim gap is preferable. For example, if the shims were to be produced by a numerically controlled machining operation, then the output from the gap measuring device could be provided in the form of an electrical signal that is manipulated by an appropriate computer interface to produce proper machine instructions. Such numerically controlled shim-producing machines exist and are in wide usage in the industry.
The age-old and still current practice is to hand fit shim blanks, i.e., pieces of suitable material in block or other three-dimensional geometric shape, to the particular gap by a cut-and-try procedure. The blanks are ready-cut to the desired pad shape from laminated sheets of the shim material. Layer after layer of the thin material is manually peeled off until the shim blank will slip into the gap. If too many layers are removed in this manner, the shim becomes useless for the given gap, and the cut-and-try procedure begins again with a new shim blank. The cut-and-try procedure is clearly subject to high material and labor costs.
Other gap-filling techniques measure the shim gap and manufacture a shim according to those measurements. Some assumptions about the characteristics of the gap are made in order to simplify this type of mechanical measuring of gap dimensions. Typically, in metal structures, the faces of the joining parts have been machine flattened. Thus, a shim gap in an assembly, such as an airplane, can be assumed to be defined by two surfaces that are generally planar. Additionally, each gap is assumed to be defined by only two planes.
In order to determine the measurements for a three-dimensional shim gap, the orientation of the surfaces defining the gap with respect to each other must be determined. This orientation can be described by the dimensions of the shim that would fill the gap space. A rigid body, or surface, in space has six degrees of freedom. In general, six values are thus required to describe the position and orientation of the body with respect to an arbitrary coordinate system. If one of the planes defining the shim gap is chosen as a reference plane, and a convenient point in that plane is the origin of a coordinate system, then the position and orientation of the other plane can be completely described with respect to the reference plane with a maximum of six measurements, the same six measurements that would define the shim. The orientation of the unknown plane is defined by angular rotations about an orthogonal axis perpendicular to the normal axis and, finally, a rotation about the normal. In practice, this last rotation is of no significance, thereby allowing the unknown plane to be adequately defined with only five measurements.
One prior art device for measuring shim gaps is a relatively large electromechanical tool. The tool includes a two-piece probe having lips that are suitable for insertion into the gap whose thickness is to be measured. The measuring portion of the device includes an electric motor coupled to the probe for moving the probe pieces apart, and a shaft angle encoder for measuring the separation between the tips of the probe. In addition to being an undesirably large and cumbersome hand tool, the device has the disadvantage of measuring the gap at a single point per insertion. Single-point measurement is undesirable because at least three gap measurements taken at very precisely located, spaced-apart positions are required in order to obtain all of the information needed to determine the planar profile of a shim. Not only is precise manual positioning expensive because it is time consuming, the multiple manual positioning of a single-point measuring device is more likely to result in errors than is the single manual positioning of a multiple-point measuring device. Additionally, mechanical devices are subject to inaccurate measurements and harm to the plane surfaces due to distortion in the surface caused by the force of the probe pieces.
Microwave, profilometer, and optical measurement devices have also been proposed to measure the thickness of interface gaps that need to be shimmed. As with the mechanical system described above, all of these proposals have the disadvantage of providing measurements at a single location per measurement. Because of the previously described difficulties associated with single-location measurement devices, it is difficult to utilize the information generated by such devices to control a machine tool system designed to automatically create a shim based on precise measurements information. In addition, many of the prior art gap-measuring devices are relatively bulky, making them unuseful when the gap is small or when the gap whose thickness is to be measured is located near adjacent structures.
This invention overcomes the drawbacks of existing shim gap measurement methods described above as well as others.